1. Field of Invention
The present invention relates to photosensitive compositions which provide highly stable colored images and to processes for preparing colored images utilizing the same. More specifically, the present invention relates to a photosensitive composition capable of dry development solely by exposure to actinic radiation or by a combination of exposure to actinic radiation and thermal development wherein either positive or negative prints can be produced.
2. Discussion of the Prior Art
Photochromic compounds and compositions that change color or intensity when exposed to light, have long been used in light sensitive photographic compositions. On exposure to light of certain wavelengths the photochromic compound undergoes a reversible color change; i.e. it becomes colored under the influence of light of certain wavelengths, whereas upon irradiation with other, usually longer wavelengths, it exhibits a tendency to revert to the colorless state. The colored images of photochromic materials also exhibit a tendency to revert to the colorless state when left in the dark or when exposed to heat.
The reversibility of the color change of photochromic compounds and compositions have been taken advantage of in various photographic or photolithographic processes. For instance, British Pat. No. 1,154,716 to Minnesota Mining and Manufacturing and U.S. Pat. No. 3,804,628 to Fuji Photo Film Company, LTD, take advantage of the transient nature of the colored image formed upon light exposure of a photochromic compound which is incorporated in photosensitive compositions containing photohardenable polymers for the preparation of lithographic plates, in order to have positive means of identifying the exposure of the photosensitive composition. In such applications the reversibility of the color forming reaction is either advantageous or at least is not disadvantageous. As noted in both of these patents a particularly preferable class of photochromic compounds are the spiropyran compounds such as the indolino spirobenzopyran derivatives. These compounds are well known in the prior art and have been fully described in such publications as "Advances in Photochemistry", volume 1, page 275ff, New York, Intersciences, 1963. Briefly, when the photochromic spiropyran compound is exposed to a source of ultraviolet radiation having a wavelength less than about 4200 A, a bond is broken and the compounds are transformed from their colorless form to the corresponding merocyanine compound, a colored form. However, the colored form is reversible and disappears upon exposure to visible light, darkness, or heat.
Naturally, the reversibility of the color change of the photochromic compounds has prevented the use of such substances in photographic compositions and photographic processes wherein it is desired to produce a permanent image. Accordingly, the prior art has developed several approaches to provide permanent colored images utilizing photochromic compounds and specifically photochromic spiropyran compounds.
For instance, Delzenne, in U.S. Pat. No. 3,730,734 describes a light sensitive photographic material comprised of a photochromic spiropyran compound and a photohardening polymeric system. Exposure of the photographic material to actinic light produces cross-linking of the photohardening polymeric systems which react with and "lock" the photochromic compound in its colored form.
It has also been proposed, for example, to form permanent color images by combining the spiropyran compound with a phenol compound. For example, Baum in U.S. Pat. No. 3,451,338 describes a thermographic method of data-recording and heat-sensitive record sheets which include chromogenic indolino-spirobenzopyran compounds and a di-phenol material which produce immediate and lasting color when and where the two co-reactant materials are brought into liquid contact, as in a melt. It has also been proposed to form permanent color images with the indolino-spirobenzopyran and other photochromic materials by locking the colored form by means of heat or metallic salts.
However, the development of these processes has not been simple and the colored images have not proven to be permanently irreversible. For instance, when phenol compounds are used the presence of untransformed phenols tend to produce a disagreeable heightening of the color resulting from the oxidation by air of the untransformed phenols to give colored quinone compounds.
French Pat. No. 1,478,155 discloses a process of selective masking, i.e. a process whereby a positive is produced from a negative wherein the variations of density of the negatives are attenuated. In this process it is suggested to use an intermediary recording material formed from a transparent support sheet coated with a spiropyran dispersed in from between 5 and 50% of hydrolyzed vinyl acetate as a binder. However, the colored form produced in this process is still only temporary, which, however, is acceptable for the overall selective masking process.
Berman in U.S. Pat. Nos. 3,072,481 and 3,090,687 describes photochromic recording materials which utilize specific photochromic spiropyran compounds which are substantially inert to color changes when in the dry and solid form. These compositions attempt to fix the colored image by dissolving the spiropyran compounds in volatile solvents which evaporate after the color has formed to leave the untransformed photochromic spiropyran compound in the dry and solid form. However, the use of volatile organic solvents is clearly objectionable with regard both to safety considerations and complex equipment requirements.
Cerreta in U.S. Pat. No. 3,212,898 describes polyester resins in which photochromic spiropyran compounds are uniformly dispersed. It was found that the photochromic properties of the spiropyran compounds was not blocked in the solid polyester resin although it was previously generally supposed that the photochromic spiropyran compounds only exhibited their photochromism when in solution. However, the reversibility of the color change was not inhibited by the solid polyester resin solutions of the photochromic compound. Thus, Cerreta teaches that the products may be transformed back to their true color from the color attained by contact with ultraviolet light by removing the exposed compositions from the light for a short period of time.
As previously mentioned the photochromic properties of spiropyran compounds has found application in lithographic printing processes wherein advantage is taken of the immediate color change upon exposure to a suitable light source. In these processes the creation of a visible image by the formation of the colored form of the spiropyran compound is taken as an indication of appropriate exposure of the lithographic plate so that the latent image may subsequently be exposed without having to worry about double exposure or other problems prevalent in the lithographic printing process. Examples of this type of application can be found in the aforementioned U.S. Pat. No. 3,804,628 to Osada et al and British Pat. No. 1,154,716 to Minnesota Mining and Manufacturing Company. The compositions described in these patents are basically conventional lithographic compositions which include oleophilic water-insoluble, solvent-softenable, photohardenable resins in which the photochromic compound is dispersed. For instance, in Example 1 of the British patent a resin millbase includes ethylene dichloride, polyvinylformal resin and epoxy resin. In Example 3 of the British Patent the coating solution composition includes methyl ethyl ketone, vinyl chloride-vinyl acetate-maleic anhydride terpolymers, methyl methacrylatestyrene copolymer, diazo-formaldehyde resin and diacetone alcohol. In Example 5 of the British patent polyvinyl cinnamate is used in a photoresist solution.
The Osada et al patent describes photosensitive compositions wherein a photochromic indolino-spirobenzopyran derivative and a sensitizer is combined with a photosensitive resin having a cinnamoyl group, a beta-(2-furyl) acryloyl group or a beta-(pyridyl) acryloyl group as side chains. The photosensitive resins containing the aforementioned side groups may be in the form of homopolymers or copolymers. Other components of the copolymers utilized in these compositions include styrene and derivatives thereof, (meth)acrylic acid or esters thereof, acrylamide, (meth)acrylonitrile, vinyl chloride, vinyl acetate, .beta.-hydroxymethylmethacrylate, N-methylolacrylamide, glycidyl methacrylate, .gamma.-chloro-.beta.-hydroxypropyl methacrylate, glycerol acrylate, and glycerol methacrylate. These materials are used at levels of from 2 to 35% by weight based on the total amount of the polymer.
However, none of these previously described compositions containing photochromic spiropyran compounds have been completely successful in forming permanent colored images in a dry development process.